1. Field of the Invention
The present invention is related to the field of electronic boards and, in particular, to means for setting up and taking apart the components thereof through soldering.
2. Description of the Related Art
An electronic board comprises overall a planar backing, referred to as a printed circuit consisting in a stack of conductors, so-called layers, and separated one from another by electrical insulators. Electronic components are mounted on such a backing and the electrical connections are provided by metal tabs being soldered on plotting lands on the printed circuit. The backing is commonly referred to in the field by the acronym PCB (for printed circuit board).
Conventional components, being of the peripheral output type, i.e. having their connection tabs arranged on the periphery of the housing containing the electronic chip, can be easily soldered to the substrate. The heat required for the solder melting and the soldering operation could be implemented by bringing the source of heat closer to the peripheral tabs without any risk of damaging the component through excessive heating.
The technological development makes it now possible to use more compact components with so-called surface outputs. There is illustrated on FIG. 1 a diagram representing a component 10 of a type referred to in the field as BGA (ball grid array). The component comprises an electronic chip 11 embedded into an encapsulation material 12, and on the lower part a substrate 13. Plotting balls 15 are integral with the substrate on the other side and are arranged as a grid. Part of the balls 15A is electrically connected with the chip 11 through appropriate electrical connecting means 16A. Some balls 15B, located in the central part of the substrate, are electrically and thermally connected with the chip through connecting means 16B for draining the calories thereof from the housing to the PCB.
Compared to the components with peripheral outputs, surface output components have the inconvenience of requiring them to be heated to make them soldered, as the balls are not otherwise available. For soldering or unsoldering such components, it is therefore required to heat the housing at the melting temperature of the soldering alloy, such as 195° C. for lead based alloys. It should be ensured that the temperature remains lower than the chip holding temperature, which is 250° C.
Additionally, in some aeronautics applications, electronic boards are enclosed in housings in which no cooling air circulates. One or more metal foils functioning as thermal drains (generally made of copper) is/are inserted into the backing, referred to as the PCB, so as to be able to drain heat through conduction between the heating electronic component(s) and the outside of the metal housing containing the electronic boards. Furthermore, in particular for aeronautics applications, a weather resistant lacquer covers the whole board, and protects it against moisture.
Because of the high cost of electronic boards in some applications, for example aeronautics, it is important to be able to repair them by replacing the defective components. Replacing the defective components should occur without disturbing adjacent components. A difficult step involves locally providing a sufficient amount of heat for resulting in the soldering melting of the only component to be replaced.
The presence of the thermal drains and the weather resistant lacquer, in such a setting, is an inconvenience when it comes to replacing a BGA type component, i.e., to unsoldering and re-soldering it from its backing.
The prior art common solutions are not appropriate for soldering a new component or for unsoldering an existing component when the backing comprises thermal drains. In this case, it is necessary to provide a higher amount of heat for accounting for its removal through the drain. Such an increase in heat is detrimental to the adjacent components. Furthermore, the weather resistant lacquer is able to melt in turn at the heating temperature. It is likely to migrate into the adjacent solders and to damage electrical connections.
The Applicant have provided a solution in the Patent Application FR 2,864,419. Such a method for soldering an electronic component under repair on a backing comprising at least one thermal drain for said component, by means of a soldering paste incorporating a stripper flux activated at a first temperature, and a soldering alloy melting at a second temperature higher than the first one, comprises the following steps:                preheating the backing through the thermal drain up to said first temperature,        positioning the component on the backing with the soldering paste, and        heating the component by applying a hot gas at a high enough temperature for activating the flux and bringing the soldering alloy to the second temperature.        
A soldering paste used in such a type of method commonly comprises a soldering alloy generally in the form of balls, a very light activating flux for processing surfaces, and a thinner or a solvent. Through heating across the thermal drain of the backing during the preheating operation, the heat supply is limited to the one required for activating the flux. The temperature of the backing and the adjacent components remain lower than that of the re-melting of the soldering alloy of the latter. Moreover, the heat required for the soldering alloy to reach the melting temperature is locally provided by the hot gas nozzle above the component. Preheating the board occurs through convection with a heating plate arranged under the board on the side opposite to the subject component.
Heating the component with hot air comprises a preheating step at an intermediate temperature, lower than the second temperature. The board is placed in an enclosure at least during the preheating step, allowing for the temperatures to be homogenized on the board. The hot gas is applied by means of a nozzle arranged perpendicularly to the plane of the board, associated with a gas return chimney forming means after the component has been swept.
In the above-noted Patent Application, there is also disclosed an assembling and repairing facility for electronic boards, for implementing the previous method(s), comprising a means for securing said board, a means for heating by thermal conduction on one side of the board and a means consisting in a heating plate for heating with hot gas on the other side of said board. Such a facility both provides for implementing the above described soldering method and for implementing the unsoldering of a component to be replaced without disturbing adjacent components.
The method is quite appropriate when the alloy to be used is based on lead as the tin lead alloy having the melting temperature able to be adjusted to 195° C. It is no longer adapted to soldering with an alloy not comprising lead, abiding to environmental regulations, and having its melting temperature higher and substantially closer to the limit temperature to which the component could withstand. Thus, an alloy such as made of tin, copper and silver, that satisfies regulation requirements, has a melting temperature of 230° C., higher by 35° C. than the above mentioned temperature, but essentially close to the 250° C. limit for the components.